Flat panel displays are widely used in a variety of applications, including computer displays. One type of device well-suited for such applications is the field emission display. Field emission displays typically include a generally planar substrate having an array of projecting emitters. In many cases, the emitters are conical projections integral to the substrate. Typically, the emitters are grouped into emitter sets where the bases of the emitters are commonly connected. The term emitters will be used herein to refer to single emitters or emitters grouped into sets.
A conductive extraction grid is positioned above the emitters and driven with a voltage of about 30-120 V. The emitters are then selectively activated by providing a current path from the bases to the ground. Providing a current path to ground allows electrons to flow to the emitters by the extraction grid voltage. If the voltage differential between the emitters and extraction grid is sufficiently high, the resulting electric field extracts electrons from the emitters.
The field emission display also includes a display screen mounted adjacent the substrate. The display screen is formed from a glass plate coated with a transparent conductive material to form an anode biased to about 1-2 kV. A cathodoluminescent layer covers the exposed surface of the anode. The emitted electrons are attracted by the anode and strike the cathodoluminescent layer, causing the cathodoluminescent layer to emit light at the impact site. The emitted light then passes through the anode and the glass plate where it is visible to a viewer.
The brightness of the light produced in response to the emitted electrons depends, in part, upon the rate at which electrons strike the cathodoluminescent layer, which in turn depends upon the magnitude of current flow to the emitters. The brightness of each area can thus be controlled by controlling the current flow to the respective emitter. By selectively controlling the current flow to the emitters, the light from each area of the display can be controlled and an image can be produced. The light emitted from each of the areas thus becomes all or part of a picture element or "pixel."
Anodes of such displays typically draw low currents on the order of just a few microamperes while operating at anode voltages of 1-2 kV. Measuring such low currents at such high voltages can require very expensive equipment. It can therefore be impractical to use available equipment to measure the anode current of field emission displays in a production environment where large numbers of field emission displays are tested repeatedly at reasonable speed and affordable cost.
The current to a single anode can be determined by monitoring current flow through the low voltage side of the voltage supply providing the anode voltage. However, such an approach becomes undesirable where a single high voltage supply is used to test several displays, because it can be difficult to determine the portion of the current attributable to each individual display.